Push-in spinal fusion implants (allowing for the growth of bone from adjacent vertebral body to adjacent vertebral body through the implant) having upper and lower surfaces adapted for placement by linear insertion within a disc space and in contact with the adjacent vertebral bodies are known in the related art. Such a push-in spinal fusion implant was invented by Michelson and is disclosed in U.S. Pat. No. 5,776,199, filed Jun. 28, 1988, which is hereby incorporated by reference. Push-in spinal fusion implants offer the advantage of being easily positioned in the implantation space and of having the ability to have varying height to width ratios.
Lordotic or tapered, push-in spinal fusion implants are also known in the art. By way of example, Michelson has invented such implants as disclosed in U.S. Pat. No. 5,609,635, filed Jun. 7, 1995, which is hereby incorporated by reference. Lordotic or tapered, spinal fusion implants may more easily restore or enhance spinal lordosis.
Spinal fusion implants having projections that can be deployed after the implant has been inserted into the disc space are also known in the related art. An example of a spinal fusion implant having deployable projections was invented by Michelson and also is disclosed in U.S. Pat. No. 5,776,199 previously incorporated by reference herein. Other examples of implants having deployable projections include, but are not limited to, U.S. Pat. No. 6,179,873 to Zientek and International Publication No. WO 01/01894 A1 to Bolger et al. Examples of spinal fusion implants having rotatable elements for fixing the implant to the vertebrae include U.S. Pat. No. 6,210,442 to Wing et al., U.S. Pat. No. 6,090,143 to Meriwether et al., and U.S. Pat. No. 5,888,228 to Knothe et al.
None of the related art implants have a rotatable internal member with bone engaging projections that are retracted within the interior of the implant to permit the implant to be inserted into the disc space and then deployed to extend through the exterior of the implant to penetrably engage the adjacent vertebral bodies, while permitting bone growth from adjacent vertebral body to adjacent vertebral body through the interior of the implant and the interior of the internal rotatable member substantially unimpeded by the internal rotatable member and bone engaging projections.
There exists a need for a spinal fusion implant providing for all of the aforementioned features in combination.
In accordance with the present invention, as embodied and broadly described herein, there is provided a spinal fusion implant for implantation at least in part within and across the generally restored height of a disc space between two adjacent vertebral bodies of a human spine having an external housing with a substantially hollow internal rotatable member having bone engaging projections that are in a retracted position within the interior of the housing to permit the assembled implant to be inserted into the disc space. The internal rotatable member may be inserted into the housing prior to insertion of the implant into the disc space, or alternatively, the housing may be inserted into the disc space and the internal rotatable member can be subsequently inserted into the housing. The implant is preferably inserted into the disc space by linear insertion without substantial rotation of the implant. Alternatively, the implant be can be rotated at least in part generally less than 180 degrees during its implantation into the disc space and is not screwed into the disc space. After insertion, the internal rotatable member is rotated to a deployed position so that the bone engaging projections extend through the exterior of the housing to penetrably engage the adjacent vertebral bodies to resist expulsion of the implant from the disc space, to gain access to the more vascular bone of the vertebral bodies further from the bone surfaces adjacent the disc space, to stabilize the adjacent vertebral bodies relative to the implant, and to stabilize the vertebral bodies relative to each other. The spinal implant is configured to permit bone growth from adjacent vertebral body to adjacent vertebral body through the housing and through the interior of the internal rotatable member preferably substantially unimpeded by further internal mechanisms.
In one embodiment, the spinal implant of the present invention has an external housing having a hollow interior and a substantially hollow rotatable member therein. The implant and each of the hollow components, that is the housing and the internal rotatable member, are adapted to hold fusion promoting substances, such as but not limited to bone. The housing preferably has relatively thin walls, openings, and except for the openings preferably a relatively smooth exterior. The rotatable member has an open interior configured to hold bone growth promoting material and at least one aperture therethrough in communication with the open interior to permit for the growth of bone therethrough. The rotatable member preferably, but not necessarily, has a generally cylindrical or frusto-conical configuration, is preferably thin-walled, and is preferably in contact with the external housing, but is free to rotate therein sufficient for its intended purpose. The rotatable member has bone engaging projections adapted to penetrably engage the bone of the adjacent vertebral bodies by rotating the rotatable member. The rotatable member is adapted to rotate within the hollow interior of the implant between a retracted position and a deployed position. The bone engaging projections extend through at least some of the openings in the upper and lower surfaces of the implant so as to penetrate the vertebral bodies adjacent the disc space to be fused deep to the adjacent superficial endplate surfaces when deployed.
In a preferred embodiment, the bone engaging projections have a blade-like configuration oriented transverse to the longitudinal axis of the rotatable member with a leading edge and a trailing edge angled relative to each other to form an apex adapted to penetrate the bone of a vertebral body. The bone engaging projections are preferably oriented on opposite sides of the rotational member and may, but need not, be diametrically opposite one another. The bone engaging projections may be arranged such that at least the apexes of two opposite bone engaging projections are on opposite sides of a mid-line passing therethrough. Such an over-center arrangement of the bone engaging projections creates a more stable configuration of the implant when the bone engaging projections are fully deployed. Greater energy is required to de-rotate the rotatable member with opposed bone engaging projections in an over-center arrangement as the apex of each bone engaging projection has to be moved through the mid-line to move from a deployed to a retracted position.
The spinal implants of the present invention may have upper and lower surfaces that are generally parallel or angled relative to one another. The spinal implants of the present invention may have a cross-section transverse to the longitudinal axis of the implant that is generally square, rectangular, or any other configuration suitable for its intended purpose. The spinal implants of the present invention may have the width equal to the height, the width greater than the height, or the width less than the height. The spinal implants of the present invention may have more than one rotatable member with bone engaging projections. The rotatable member can have a generally cylindrical configuration, a generally frusto-conical configuration, or any other configuration suitable for the intended purpose.
The present invention is also directed to an implant inserter instrument adapted to insert the spinal implant into an implantation site and deploy the bone engaging projections. The inserter instrument is configured to cooperatively engage the trailing end of the implant to rotate the rotatable member to deploy the bone engaging projections. The present invention is also directed to the methods for inserting and deploying a spinal implant in accordance with the present invention.